28 THE OVUM OR EGG. 



There is a white circular area, a little more than the 

 eighth of an inch in diameter, which from being sur- 

 rounded hy the lightest part of the yolk, is always 

 uppermost as the egg lies. In the centre of this is the 

 minute mass of bioplasm which is the parent of all 

 those from wliich the tissues are subsequently formed. 

 Now in batrachia (frogs, newts, &c.) and fishes the 

 part of the ovum which corresponds to the great bulk 

 of the egg or the food yolk of the bird is very small, 

 because the pabulum required during development is 

 principally derived from the surrounding fluid, which 

 is filtered by passing through a most delicate fibril- 

 lated tissue, which surrounds the ovum.* In mam- 

 m.alia the ovum is very minute, and consists almost 

 entirely of bioplasm, which takes part in the formation 

 of the germ. Its nutrient matter is supplied from 

 a soft spongy tissue of the mother, specially formed 

 for it. In this it is embedded. It imbibes its pabu- 

 lum during the early period of its existence. As it 

 grows it derives the substances to nourish it from the 

 mother's blood (§ 58). The germ is not in any 

 way dependent upon nntrient matter formed with it 

 or soon after it, and inclosed with it in a confined space, 

 as in the case of the eggs of birds, and also in those of 

 the snake, tortoise, and many other members of the 

 reptilia. The mammalian germ is in fact entirely and 

 solely dependent upon the maternal organism for its 

 nutrition. Moreover the eggs of most reptiles and 

 some birds are hatched altogether without the assist- 

 ance of the mother, and in few cases is this a necessary 

 condition of development. The developmental changes 

 proceed quite as regularly and as perfectly when the 

 egg is exposed to artificial heat as when it is incu- 



* This looks lite the white or albumen of the bird's egg, but 

 it is not albiiminous, find consists of very delicate tissue, the 

 meshes of which are occupied by water. It is a structure Tery 

 like the vitreous liumour of the eye. 



